Finite Element Method Numerical Simulation Of Forest Roots Reinforcement

The species distribution model for Robinia pseudoacacia and Pinus tabulaeformis was established in the Loess Plateau based on Soil Mechanics, Elasto-plastic Mechanics, Soil & Water Conservation, Forest Ecology and Mathematics theories. A Pinus tabulaeformis roots constitutive model was built up based on the root system stress-strain relationships, and a interface constitutive model for Robinia pseudoacacia and Pinus tabulaeformis was set up by testing direct shear of the interface elements friction characteristics. The effects of horizontal, vertical and complex root arrangement on stress-strain of root-soil composite was tested, and the root-soil composite displacement and stress fields were stimulated by using finite element method. On analyzing the effects of afforestation, tree species, planting density, slope shape and precipitation on slope stress-strain relationship, the interaction mechanism between roots and soils was revealed, and the effect of planting density, stand age, slope shape and precipitation on slope stability were analyzed with Swedish slice method. All research results provided theoretical basis for the prevention of soil corrosion, slope treatment and planting optimal design.The main results are summarized as following:1.Pinus tabuleeformis is of vertical roots while Robinia pseudoacacia is of horizontal. Both trees'horizontal root basal diameter, main root, root biomass and diameter at breast height coordinate with power function relationship. Based on Fitter and Rose description and rule, the root distribution model of Robinia pseudoacacia and Pinus tabulaeformis was established.2.The root tensile test shows that, the tensile properties of Pinus tabuleeformis roots were elastic deformation and the stress and strain were linear during the early stage, when the stress beyond ultimate tension by 50%-70%, they would be plastic deformation and non-linear relationship respectively. The factors effecting root tensile properties include root length, root diameter, root shape, etc. Based on analyzing the stress-strain relationship, hyperbola constitutive model, second-order parabola model and third-order parabola model were built up Pinus tabulaeformis root system.3. The experiment of interface friction between roots and soils shows that, main factors effecting root-soil interface frictional resistance were soil density, soil water content, embedment and root characteristic. A significant positive relationship exists between the ratio of root-soil interface friction coefficient to soil friction coefficient and soil density, and a significant negative relationship between root-soil interface friction coefficient and soil water content. The root-soil interface friction coefficient increases with root buried depth. Roots roughness effects on root-soil interface friction. Robinia pseudoacacia has stronger effects on slope stability than Pinus tabuleeformis. Meanwhile, a root-soil interface constitutive model was developed and the parameters were tested out through direct shear friction.4. The root-soil composite intensity test revealed the effects of horizontal, vertical and complex roots arrangement, and determined root-soil composite constitutive model and its parameters. The complex root system has the strongest reinforcement, and then is the vertical root system, the horizontal one is the lowest. Main factors effecting root-soil composite intensity were root type, root amount, soil water content and soil density, etc. The test proved the root function of reinforcement and decreasing slope pore pressure.5. Regarding root-soil complex as an organism with combines soil, root system and interface between root and soil, the stress and deformation characteristic of afforestation slope, discussed the effect of afforestation, tree species, planting density, slope shape and precipitation on slope stability innovationally were analyzed by using finite element method. The finite element method analysis results show that planting trees on the top and toe of slope properly in line of the climate and soil conditions of the research area could decrease tensile stress range on the top of slope and the shearing stress range on the toe of slope. The finite element method and Swedish slice method show that afforestation slope stability decreases with the increasing slope angle and slope length, and roots could absorb water in slope, make soil water content of non-saturated region in slope increase gradually, and improve the slope stability.6. It is found that roots reinforcement takes place mostly in the shallow layer within 0-5m area, and the reinforcement is not obvious in the area over 5m. It means that vegetation measure is suitable for low slope stability and shallow landslide control, but unfit for high slope and deep landslide.7. The finite element method can stimulate the root-soil interaction, shows the function that forest is capable to transfer superficial soil stress to deeper and reduces superficial stress through root. In the practice of afforestation, the finite element method can be used to analyze the stress-strain relationship of afforestation slope quantitatively and help to choose suitable tress species and planting density.